1. Field of the Invention
The present invention relates to a method and system for transmitting a halftone image, and, in particular, to a method for handling an image signal representative of a halftone image employing the dither method to be transmitted between a transmitter and a receiver at a remote place under compression.
2. Description of the Prior Art
In transmitting an image signal, use is normally made of MH (Modified Huffman) coding or MR (Modified Read) coding according the CCITT standards so that the transmission of binary image signal can be carried out at high efficiency with considerable suppression of redundancy. Although use is made of either of these coding schemes, when transmitting a halftone image, because of peculiarities present in a halftone image, data compression cannot be carried out efficiently, and, in some cases, the amount of data after compression becomes larger than that before compression. It has thus been recognized that impracticability exists in having a halftone image signal coded in the same manner as in the case of having a binary image signal coded. This is a natural consequence because the MH and MR coding schemes mentioned above have been set up so as to be consistent with the statistical properties of typical documentary or graphical images, which may be converted into binary image data quite simply. Since a halftone image, such as a picture, has a gray scale continuously changing in density from black to white as opposed to documentary images, the coding efficiency may drop dramatically when the MH and MR schemes, which have originally been established for documentary images, are used.
According to CCITT standards, for such a special image as a halftone image, there is provided a "non-compression" mode in which a string of pixel data is transmitted without compression. This is because, there are cases in which the amount of data may be maintained smaller if the data is not coded. This is not always advantageous because in the case of representing halftone with the use of the dither method, it may be so devised that compression coding can be carried out effectively by utilizing its periodic nature.
Use has often been conventionally made of the systematic dither method in order to represent a halftone image by binary pixels. According to this method, however, since the threshold for comparison with the density level of pixel in a halftone image to be processed is used periodically with the dither matrix as a unit in repetition, if an image having a periodic pattern is processed, for example, by a mesh-point image, there is often produced a significant moire in the resulting image. This is caused by the conflict between the periodicity of the threshold used and the periodicity in pattern of the input image, and as long as use is made of the systematic dither method, it is virtually impossible to prevent this from occurring. In order to cope with this, use may be made of the random dither method, in which a distribution of thresholds in the dither matrix is set up using random numbers; however, this method is not used so often because the quality of resulting image is relatively poor.
Several methods for preventing moire from occurring in the dither method are proposed as disclosed in Japanese Patent Laid-open Pub. Nos. 57-60772 and 57-125579. In accordance with the method disclosed in the former publication, the properties of an image to be processed are detected by the degree of changes in image signal between a pixel of interest and its surrounding pixels according to the contents stored in a buffer shift register and the pixel of interest is subjected to binary value conversion while changing the threshold level depending on the detected information, thereby carrying out the random dither processing to decrease the periodic moire produced in the binary-valued image signal. On the other hand, in accordance with the method disclosed in the latter publication, using the random number generated by a random number generator as the threshold value, the density level of each of the pixels of a halftone image is compared thereby determining the density level of output image signal to be either white or black level, whereby the difference between the density level of the pixel of interest for comparison and the density level of the output image is added as distributed in accordance with the coefficient specific to the density level of each of a plurality of pixels adjacent to the pixel of interest prior to comparison to correct the density level of these pixels thereby allowing to prevent moire from being produced in the output image.
In this manner, when representation of a halftone image is to be carried out for an image having a high frequency periodic pattern, use of either of the above-mentioned two schemes allows to decrease production of moire, but there is produced a deterioration of image quality.
Under the circumstances, the present inventor has proposed an improved method as disclosed in Japanese Patent Application No. 58-214683 (now Laid-open Pat. No. 60-106269), entitled "HALFTONE IMAGE TRANSMISSION SYSTEM", which allows not only to eliminate moire but also to represent halftone high in quality by emphasizing the peripheral portion (contour) between dark and light, which significantly influences the quality of a halftone image, at the time of coding for transmission of image data. According to the proposed method, the average value of density level is obtained for the m.times.n pixel region corresponding to the m.times.n dither matrix as a unit and the thus obtained average value is transmitted as a representaive or sample value. That is, according to this method, the image information is transmitted with the sampling of data which is coarser than the sampling of data with the original pixel as a unit.
This is because, in the case of transmission of a halftone image having no abrupt changes in density, no major error will be produced when considered the nature of the dither method. However, even in light and dark information, the contour portion is dynamic in the change between light and dark, and the position of such a boundary must be transmitted accurately without causing any blur. Nevertheless, according to the above-described method, since sampling is made coarser, the information as to the position of boundary becomes obscure as compared with the customary halftone image transmission method. Reproducibility may be improved to some extent by applying the MTF (Modulation Transfer Function) correction for each unit region, but there is a structural limitation in the above-described method because the unit region to be corrected is already coarse so that there are some cases in which distortion appears in the contour since the unit region to be corrected is relatively large.
Therefore, there has been a need to provide a scheme of halftone image transmission high in compression efficiency without lowering the sampling efficiency significantly.